1. Field of the Invention
The present invention relates to a flat type cable and, more particularly to a flat type cable suitable for use in high frequency applications.
2. Description of Related Art
Twisted pair cables are widely used as high-speed data communications media. One common type of conventional cable for high-speed data transmission includes four twisted pairs that may be bundled and twisted (cabled) together and further covered with a jacket to form the cable. Typically, two of the twisted pairs may transmit data and the other two of pairs may receive data.
A transmission cable utilizing above described twisted pair technology must meet particular specifications with respect to certain electrical characteristics for transmission at high frequencies. The electrical characteristics include controlled impedance, controlled near-end cross-talk (NEXT), controlled equated level far end cross talk (ELFEXT), controlled attenuation, and so on. These specification requirements may become more stringent for performance at higher frequencies. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) has developed standards providing such specifications for high performance transmission cables. The International Electrotechnical Commission (IEC) has also defined standards for the same. TIA's Category 6 cable, commonly referred to as Cat-6, is a cable standard that provides specifications for performance up to 250 MHz and is suitable for networks like 10 BASE-T, 100 BASE-TX (Fast Ethernet), 1000 BASE-T/1000 BASE-TX (Gigabit Ethernet), and specifications for performance up to 500 MHz for mitigated 10 GBASE-T (10-Gigabit Ethernet). Compared with Cat-5 and Cat-5e cable standards, Cat-6 features more stringent specifications for crosstalk and system noise. Similarly, the Cat-7 cable standard which is defined for frequencies up to 1000 MHz features more stringent specifications for crosstalk and system noise compared to Cat-6.
For better crosstalk performance, in general, twisted pair conductors of different lay lengths are used inside a transmission cable. In ordinary transmission cables which have a round cross section, twisted pairs are arranged in an alternate Long, Short, Long, Short (LSLS) lay configuration. In other words, the twisted pairs of long lay length and short lay length are arranged alternately within the circumference of the circular cross section of the cable in a substantially circular arrangement. However, cables with a round cross section have a greater number of possible crosstalk combinations vis-à-vis a flat type cable. In a flat type cable, the multiple twisted pairs are laid out in a substantially flat arrangement, for example in a coplanar or crescent shaped arrangement, inside the jacket.
In addition to having fewer closely spaced possibly troublesome crosstalk combinations, which is combinations that at the operating frequencies experience enough crosstalk to affect the performance of the cable, flat type cables have other favorable properties. For example, flat type cables have a smaller bend radius across the minor cable axis, and have the ability to roll up compactly. Flat type cables are also preferred due to the elimination of the overall strand operation requirement in cable layout. Some implementations of such flat type cables are described in U.S. Pat. No. 5,821,467. Known flat type cables such as Belden's MediaTwist™ also use a LSLS lay configuration, that is, they have an alternate arrangement or long lay and short lay twisted pair conductors. However, using the LSLS configuration in flat cables has one or more disadvantages.
First, in this configuration, one of the edge twisted pairs is a short lay pair. Short lay twisted pair conductors typically experience greater attenuation than long lay twisted pair conductors. In addition, the edge twisted pair conductors in a flat cable suffer more attenuation compared to central ones as the edge pairs are surrounded by more jacket material.
Owing to the aforementioned factors, the short lay twisted pair conductor on the cable's edge experiences more attenuation than the other twisted pair conductors in the cable, and becomes the cable's physical and electrical performance bottleneck. Second, one of the central twisted pairs is a long lay pair. Long lay twisted pair conductors typically show poorer crosstalk performance than the short lay pairs. Further, the central twisted pair has two close physical proximity crosstalk combinations, as against a single close physical proximity crosstalk combination of an edge pair. These factors, in combination, lead to suboptimal attenuation and crosstalk performance of flat cables using the LSLS configuration, particularly at higher frequencies.